Re: [PATCH] 2.4: variable HZ

[Neil Conway <nconway.list@ukaea.org.uk>]

Hiya...  Nice patch, must try it when I find a few minutes - I think I
have a few apps that are limited by select/poll behaviour.

I was looking at your jiffies_to_clock_t() macro, and I notice that it
will screw up badly if the user chooses a HZ value that isn't a multiple
of the normal value (e.g. 1000 is OK, 512 isn't).

How about tweaking the macro a little?  Instead of:
x / (HZ/USER_HZ)
you could use:
(x/HZ*USER_HZ + x%HZ*USER_HZ/HZ)

which minimises roundoff error and also won't overflow (at least, it
won't overflow as long as HZ*USER_HZ doesn't overflow!).

You could even use both versions of the macro, choosing between them at
compile time depending on whether or not (HZ % USER_HZ == 0).

What do you think?

cheers
Neil
PS: I'm off-list.